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134 NATIONAL GEOGRAPHIC * JULY 2006
Was walking on two legs an
energy-saving measure for our
ancestors? To help scientists
fathom the answer, a trained,
ten-year-old chimpanzee
named Louie ambles willingly
on a treadmill, first on two
legs, and then on all four, his
more usual "knuckle-walking"
gait. The loose-fitting flow
through mask captures his
exhalations, which scientists
analyze to measure energy
expenditure. White dots on his
joints convey to cameras the
location and movement of his
limbs in space. By revealing
the amount of energy Louie
expends with each step and
how it's used, such experi
ments may shed light on the
energetics of our chimplike
forebears-and whether ener
gy economy was a likely cata
lyst for their shift to bipedalism
several million years ago.